1. Field of the Invention
This invention relates to abrasive particles, and, more particularly, a method of making abrasive particles having specified shapes.
2. Discussion of the Art
Three basic technologies that have been employed to produce abrasive grains having a specified shape are (1) fusion, (2) sintering, and (3) chemical ceramic.
In the fusion process, abrasive grains can be shaped by a chill roll, the face of which may or may not be engraved, a mold into which molten material is poured, or a heat sink material immersed in an aluminum oxide melt. U.S. Pat. No. 3,377,660 discloses a process comprising the steps of flowing molten abrasive material from a furnace onto a cool rotating casting cylinder, rapidly solidifying the material to form a thin semisolid curved sheet, densifying the semisolid material with a pressure roll, and then partially fracturing the strip of semisolid material by reversing its curvature by pulling it away from the cylinder with a rapidly driven cooled conveyor, whereupon the partially fractured strip is deposited onto a collector in the form of large fragments, which, upon being rapidly cooled and solidified, break up into smaller fragments capable of being reduced in size to form conventional abrasive grains. U.S. Pat. Nos. 4,073,096 and 4,194,887 disclose a process comprising the steps of (1) fusing an abrasive mix in an electric arc furnace, (2) dipping a relatively cold substrate into the molten material, whereby a layer of solid abrasive material is quickly frozen (or plated) on the substrate, (3) withdrawing the plated substrate from the molten material, and (4) breaking the solidified abrasive material away from the substrate and collecting it for further processing to produce abrasive grains.
In the sintering process, abrasive grains can be formed from refractory powders having a particle size of up to 10 micrometers in diameter. Binders can be added to the powders along with a lubricant and a suitable solvent, e.g., water. The resulting mixtures, pastes, or slurries can be shaped into platelets or rods of various lengths and diameters. The resulting shaped grains must be fired at high temperatures, e.g., 1,400.degree. C. to 1,800.degree. C., at high pressures, or for long soak times, e.g., up to 10 hours. Crystal size may range from under one micrometer up to 25 micrometers. To obtain shorter residence times and/or smaller crystal size, either the pressure or temperature must be increased. U.S. Pat. No. 3,079,242 discloses a method of making abrasive grains from calcined bauxite material comprising the steps of (1) reducing the material to a fine powder, (2) compacting under affirmative pressure and forming the fine particles of said powder into grain sized agglomerations, and (3) sintering the agglomerations of particles at a temperature below the fusion temperature of the bauxite to induce limited recrystallization of the particles, whereby abrasive grains are produced directly to size. U.S. Pat. No. 4,252,544 discloses alumina abrasive grains produced by sintering wherein the grain structure is constructed of alumina coarse crystal particles and alumina fine crystal particles located between the alumina coarse crystal particles. U.S. Pat. No. 3,491,492 discloses a process for making an aluminous abrasive grain formed from bauxite or mixtures of bauxite and Bayer process alumina wherein the comminuted aluminous material is mixed with water and ferric ammonium citrate, or with ferric ammonium citrate and citric acid, and reduced to a state of fine subdivision by milling to give a fluid slurry of high solid content, drying said slurry to coherent cakes having a thickness equal to one dimension of the final grain before sintering, breaking said cakes to grains, screening, optionally rounding said grains by air mulling, screening, sintering, cooling, and screening to yield the final product. U.S. Pat. No. 3,637,630 discloses a process in which the same type of slurry disclosed in U.S. Pat. No. 3,491,492 is plated or coated on a rotating anode of an electrophoretic cell. The plated aluminous material is removed from the rotating anode, dried, broken to granules, screened, sintered, and screened to final size.
Chemical ceramic technology involves converting a colloidal dispersion or hydrosol (sometimes called a sol), optionally in a mixture with solutions of other metal oxide precursors, to a gel or any other physical state that restrains the mobility of the components, drying, and firing to obtain a ceramic material. A sol can be prepared by any of several methods, including precipitation of a metal hydroxide from an aqueous solution followed by peptization, dialysis of anions from a solution of metal salt, solvent extraction of an anion from a solution of a metal salt, hydrothermal decomposition of a solution of a metal salt having a volatile anion. The sol optionally contains metal oxide or precursor thereof and is transformed to a semi-rigid solid state of limited mobility such as a gel by, e.g., partial extraction of the solvent, e.g., water. Chemical ceramic technology has been employed to produce ceramic materials such as fibers, films, flakes, and microspheres. U.S. Pat. No. 4,314,827 discloses synthetic, non-fused aluminum oxide based abrasive mineral having a microcrystalline structure of randomly oriented crystallites comprising a dominant continuous phase of alpha alumina and a secondary phase. U.S. Pat. No. 4,744,802 discloses an abrasive grain made by a chemical ceramic process that employs an iron oxide nucleating agent to enhance the transformation to alpha alumina. This patent also suggests that the gel can be shaped by any convenient method such as pressing, molding, or extruding. U.S. Pat. No. 4,848,041 discloses a shaped abrasive grain made by a chemical ceramic process in which the abrasive grain has a mean particle volume ratio of less than 0.8.